This invention relates to the hydroconversion of carbonaceous materials such as coal and petroleum residua and is particularly concerned with a method for decreasing and controlling the viscosity of the high molecular weight bottoms streams produced in such hydroconversion processes.
Processes for the hydroconversion or liquefaction of coal and similar carbonaceous solids normally require contacting of the solid feed material with a hydrocarbon solvent and molecular hydrogen at elevated temperature and pressure to break down the complex high molecular weight starting material into lower molecular weight hydrocarbon liquids and gases. One of the most promising processes of this type is carried out with a hydrogen-donor solvent, which gives up hydrogen atoms in reaction with organic radicals liberated from coal or other feed material during the hydroconversion or liquefaction step. Within the hydroconversion or liquefaction zone, the high molecular weight constituents of the coal are cracked and hydrogenated to form lower molecular weight vapors and liquid products. The effluent from the liquefaction reactor is then separated into gases, relatively low molecular weight liquids and one or more bottoms stream containing higher molecular weight liquids, unconverted carbonaceous material and mineral matter.
The viscosity of the bottoms streams produced in coal hydroconversion or liquefaction processes tends to be relatively high because the bottoms streams are composed of high molecular weight constituents and mineral matter. In order to pump the bottoms streams produced by subjecting the liquefaction zone effluent to one or more separation steps, the viscosity of the bottoms streams must be maintained below an upper limit of about 150 poise. As conversion in the liquefaction zone increases, the organic content of the bottoms decreases and the mineral matter content increases. Since the contribution to viscosity of the inorganic or mineral matter fraction in the bottoms will increase as conversion in the liquefaction zone increases, it is necessary to decrease the viscosity contribution of the organic fraction in order to maintain the overall viscosity of the bottoms at a relatively low value. A decrease in the viscosity contribution of the organic portion of the bottoms will in turn allow conversion in the liquefaction zone to be carried out to a greater degree thus increasing the amount of desirable products and decreasing the amount of high molecular weight bottoms constituents.
Unlike the bottoms streams produced in coal hydroconversion or liquefaction processes, the bottoms streams produced in petroleum residuum hydroconversion processes will contain little, if any, mineral matter. These bottoms streams will, however, contain high molecular weight liquids and unconverted carbonaceous material and will therefore tend to have relatively high viscosities. Thus, it may be desirable in some residuum hydroconversion processes to have the capability of controlling the viscosities of the bottoms streams so that they can be maintained below desired values.
In coal and residuum hydroconversion processes wherein high molecular weight bottoms streams are produced, the bottoms will contain a relatively large amount of organic material that must be utilized in some way to make the overall process economical. The bottoms could be burned to generate heat, subjected to gasification to produce hydrocarbon gases or submitted to other conversion processes. In some instances it is necessary to store the bottoms prior to their subsequent processing. This storage is normally done at elevated temperatures to keep the bottoms in a molten state and it has been found that during such storage, the bottoms viscosity may tend to increase to unacceptably high values. Methods to prevent this viscosity increase are needed in order to ensure that the bottoms can be pumped to subsequent downstream units for further processing.